Selective epitaxial formation of semiconductive films

ABSTRACT

Epitaxial layers are selectively formed in semiconductor windows by a cyclical process of repeated blanket deposition and selective etching. The blanket deposition phases leave non-epitaxial material over insulating regions, such as field oxide, and the selective etch phases preferentially remove non-epitaxial material while deposited epitaxial material builds up cycle-by-cycle. Quality of the epitaxial material improves relative to selective processes where no deposition occurs on insulators. Use of a germanium catalyst during the etch phases of the process aid etch rates and facilitate economical maintenance of isothermal and/or isobaric conditions throughout the cycles. Throughput and quality are improved by use of trisilane, formation of amorphous material over the insulating regions and minimizing the thickness ratio of amorphous:epitaxial material in each deposition phase.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit under 35 U.S.C. §120 toU.S. application Ser. No. 11/536,463 filed on Sep. 28, 2006 and under 35U.S.C. §119(e) to U.S. provisional application No. 60/811,703, filedJun. 7, 2006, the disclosures of which are hereby incorporated byreference.

This application is also related to U.S. patent application Ser. No.11/343,275 (filed 30 Jan. 2006; Attorney Docket ASMEX.511A), U.S. patentapplication Ser. No. 11/343,264 (filed 30 Jan. 2006; Attorney DocketASMEX.517A), U.S. Patent Application Publication 2003/0036268 (filed 29May 2002; Attorney Docket ASMEX.317A), and U.S. Pat. No. 6,998,305(filed 23 Jan. 2004; Attorney Docket ASMEX.425A). The entire disclosureof all of these related applications is hereby incorporated by referenceherein.

FIELD OF THE INVENTION

This invention relates generally to the deposition of silicon-containingmaterials in semiconductor processing, and relates more specifically toselective formation of silicon-containing materials on semiconductorwindows.

BACKGROUND OF THE INVENTION

In forming integrated circuits, epitaxial layers are often desired inselected locations, such as active area mesas among field isolationregions, or even more particularly over defined source and drainregions. While non-epitaxial (amorphous or polycrystalline) material canbe selectively removed from over the field isolation regions afterdeposition, it is typically considered more efficient to simultaneouslyprovide chemical vapor deposition (CVD) and etching chemicals, and totune conditions to result in zero net deposition over insulative regionsand net epitaxial deposition over exposed semiconductor windows. Thisprocess, known as selective epitaxial CVD, takes advantage of slownucleation of typical semiconductor deposition processes on insulatorslike silicon oxide or silicon nitride. Such selective epitaxial CVD alsotakes advantage of the naturally greater susceptibility of amorphous andpolycrystalline materials to etchants, as compared to the susceptibilityof epitaxial layers.

Examples of the many situations in which selective epitaxial formationof semiconductor layers is desirable include a number of schemes forproducing strain. The electrical properties of semiconductor materialssuch as silicon, germanium and silicon germanium alloys are influencedby the degree to which the materials are strained. For example, siliconexhibits enhanced electron mobility under tensile strain, and silicongermanium exhibits enhanced hole mobility under compressive strain.Methods of enhancing the performance of semiconductor materials are ofconsiderable interest and have potential applications in a variety ofsemiconductor processing applications. Semiconductor processing istypically used in the fabrication of integrated circuits, which entailsparticularly stringent quality demands, as well as in a variety of otherfields. For example, semiconductor processing techniques are also usedin the fabrication of flat panel displays using a wide variety oftechnologies, as well as in the fabrication of microelectromechanicalsystems (“MEMS”).

A number of approaches for inducing strain in silicon- andgermanium-containing materials have focused on exploiting thedifferences in the lattice constants between various crystallinematerials. For example, the lattice constant for crystalline germaniumis 5.65 Å, for crystalline silicon is 5.431 Å, and for diamond carbon is3.567 Å. Heteroepitaxy involves depositing thin layers of a particularcrystalline material onto a different crystalline material in such a waythat the deposited layer adopts the lattice constant of the underlyingsingle crystal material. For example, using this approach strainedsilicon germanium layers can be formed by heteroepitaxial depositiononto single crystal silicon substrates. Because the germanium atoms areslightly larger than the silicon atoms, but the depositedheteroepitaxial silicon germanium is constrained to the smaller latticeconstant of the silicon beneath it, the silicon germanium iscompressively strained to a degree that varies as a function of thegermanium content. Typically, the band gap for the silicon germaniumlayer decreases monotonically from 1.12 eV for pure silicon to 0.67 eVfor pure germanium as the germanium content in the silicon germaniumincreases. In another approach, tensile strain is provided in a thinsingle crystalline silicon layer by heteroepitaxially depositing thesilicon layer onto a relaxed silicon germanium layer. In this example,the heteroepitaxially deposited silicon is strained because its latticeconstant is constrained to the larger lattice constant of the relaxedsilicon germanium beneath it. The tensile strained heteroepitaxialsilicon typically exhibits increased electron mobility. In both of theseapproaches, the strain is developed at the substrate level before thedevice (for example, a transistor) is fabricated.

In these examples, strain is introduced into single crystallinesilicon-containing materials by replacing silicon atoms with other atomsin the lattice structure. This technique is typically referred to assubstitutional doping. For example, substitution of germanium atoms forsome of the silicon atoms in the lattice structure of single crystallinesilicon produces a compressive strain in the resulting substitutionallydoped single crystalline silicon material because the germanium atomsare larger than the silicon atoms that they replace. It is possible tointroduce a tensile strain into single crystalline silicon bysubstitutional doping with carbon, because carbon atoms are smaller thanthe silicon atoms that they replace. Additional details are provided in“Substitutional Carbon Incorporation and Electronic Characterization ofSi_(1-y)C_(y)/Si and Si_(1-x-y)Ge_(x)C_(y)/Si Heterojunctions” by JudyL. Hoyt, Chapter 3 in “Silicon-Germanium Carbon Alloy”, Taylor andFrancis, pp. 59-89 (New York 2002), the disclosure of which isincorporated herein by reference, and is referred to herein as “the Hoytarticle.” However, non-substitutional impurities will not induce strain.

Similarly, electrical dopants should also be substitutionallyincorporated into epitaxial layers in order to be electrically active.Either the dopants are incorporated as deposited or they will need to beannealed to achieve the desired level of substitutionality and dopantactivation. In situ doping of either impurities for tailored latticeconstant or electrical dopants are often preferred over ex situ dopingfollowed by annealing to incorporate the dopant into the latticestructure because the annealing consumes thermal budget. However, inpractice in situ substitutional doping is complicated by the tendencyfor the dopant to incorporate non-substitutionally during deposition,for example, by incorporating interstitially in domains or clusterswithin the silicon, rather than by substituting for silicon atoms in thelattice structure. Non-substitutional doping complicates, for example,carbon doping of silicon, carbon doping of silicon germanium, and dopingof silicon and silicon germanium with electrically active dopants. Asillustrated in figure 3.10 at page 73 of the Hoyt article, priordeposition methods have been used to make crystalline silicon having anin situ doped substitutional carbon content of up to 2.3 atomic %, whichcorresponds to a lattice spacing of over 5.4 Å and a tensile stress ofless than 1.0 GPa.

SUMMARY OF THE INVENTION

In accordance with another aspect of the invention a method is providedfor selectively forming semiconductor material in semiconductor windows.The method includes providing a substrate within a chemical vapordeposition chamber, where the substrate comprises insulating surfacesand single-crystal semiconductor surfaces. Semiconductor material isblanket deposited over the insulating surfaces and the single-crystalsemiconductor surfaces of the substrate, such that a thickness ratio ofnon-epitaxial semiconductor material over the insulating surfaces toepitaxial semiconductor material over the single-crystal semiconductorsurfaces is less than about 1.6:1. Non-epitaxial semiconductor materialis selectively removed from over the insulating surfaces, whereinblanket depositing and selectively removing are conducted within thechemical vapor deposition chamber.

In accordance with another aspect of the invention a method is providedfor selectively forming epitaxial semiconductor material. Semiconductormaterial is blanket deposited to form epitaxial material oversingle-crystal semiconductor regions of a substrate and to formnon-epitaxial material over insulating regions of the substrate. Thenon-epitaxial material is selectively removed from over the insulatingregions by exposing the blanket deposited semiconductor material to anetch chemistry including a halide source and a germanium source. Blanketdepositing and selectively removing are repeated at least once.

In accordance with another aspect of the invention a method is providedfor forming silicon-containing material in selected locations on asubstrate. The method includes providing a substrate having exposedwindows of single-crystal semiconductor among field isolation regions.Silicon-containing material is blanket deposited over the windows ofsingle-crystal material and the field isolation regions by flowingtrisilane over the substrate. The silicon-containing material isselectively removed from over the field isolation regions. Blanketdepositing and selectively removing are repeated in a plurality ofcycles.

In accordance with another aspect of the invention a method is providedfor selectively forming epitaxial semiconductor material. The methodincludes providing a substrate with insulating regions and semiconductorwindows formed therein. Amorphous semiconductor material is depositedover the insulating regions and the epitaxial semiconductor material isdeposited over the semiconductor windows. The amorphous semiconductormaterial is selectively etched from over the insulating regions whileleaving at least some epitaxial semiconductor material in thesemiconductor windows. Blanket depositing and selectively removing arerepeated in a plurality of cycles.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of the methods and systems disclosed herein areillustrated in the accompanying drawings, which are for illustrativepurposes only. The drawings comprise the following figures, in whichlike numerals indicate like parts.

FIG. 1 is a flowchart illustrating a process for selectively formingepitaxial semiconductor layers, using the particular example ofdepositing a carbon-doped silicon film in recessed source/drain regionsof a mixed substrate.

FIG. 2 is a schematic illustration of a partially formed semiconductorstructure comprising patterned insulator regions formed in asemiconductor substrate.

FIG. 3 is a schematic illustration of the partially formed semiconductorstructure of FIG. 2 after performing a blanket deposition of acarbon-doped silicon film over the mixed substrate surface.

FIG. 4 is a schematic illustration of the partially formed semiconductorstructure of FIG. 3 after performing a selective chemical vapor etchprocess to remove carbon-doped silicon from oxide regions of the mixedsubstrate.

FIGS. 5A-5D are schematic illustrations of the partially formedsemiconductor structure of FIG. 4 after performing further cycles ofblanket deposition and selective etch.

FIG. 6 shows a graph of etch rate of amorphous regions of a carbon-dopedsilicon film as a function of HCl partial pressure in the etchchemistry.

FIG. 7 shows a graph of etch rates and ratios amorphous (“a”) and singlecrystal (“c”) etch rates as a function of GeH₄ flow in the etchchemistry for various etch chemistries.

FIG. 8 shows a graph of etch rate of amorphous regions of a carbon-dopedsilicon film as a function of chamber pressure.

FIG. 9 shows a graph of etch rate of amorphous regions of a carbon-dopedsilicon film as a function of reciprocal temperature.

FIG. 10 shows a graph of thickness of amorphous regions of acarbon-doped silicon film as a function of accumulated etch time.

FIG. 11 shows a graph of etch rate of amorphous regions of acarbon-doped silicon film deposited on a wafer as a function of radialposition on the wafer.

FIG. 12 shows a graph of thickness of amorphous regions of acarbon-doped silicon film deposited on a wafer as a function of radialposition on the wafer for various etch cycle durations.

FIG. 13 shows a graph of thickness of amorphous regions of acarbon-doped silicon film deposited of a wafer as a function of radialposition of the wafer for various GeH₄ etchant etch chemistries andvarious etch cycle durations.

FIG. 14 is a micrograph illustrating an example partially formedcarbon-doped silicon structure created by performing blanket depositionand a partial etch cycle on a patterned substrate.

FIG. 15 shows a graph of element concentration as a function of depthfor an exemplary partially formed carbon-doped silicon film formed usingcertain of the techniques disclosed herein.

FIG. 16 is a micrograph illustrating an exemplary formed carbon-dopedsilicon structure created by performing multiple deposition and etchcycles on a patterned substrate.

FIG. 17 illustrates an atomic force microscopy analysis of an epitaxialcarbon-doped silicon film that has been selectively formed using certainof the cyclical techniques disclosed herein.

FIG. 18 shows x-ray diffraction rocking curves of carbon-doped siliconfilms deposited using certain of the cyclical techniques disclosedherein.

DETAILED DESCRIPTION OF THE INVENTION

Deposition techniques often attempt to tailor the amount or kind ofdeposition in different regions of a substrate. For example, U.S. Pat.No. 6,998,305 recognizes that simultaneous etch and deposition reactionsare know for selective deposition on silicon without depositing onsilicon oxide. To control deposition on a third type of surface, namelyan exposed transistor gate, the '305 patent teaches cyclicallyalternating a selective deposition with an etch phase. However, theinventors have recognized that selective deposition chemistriessometimes have undesirably effects on the deposited layers. While thedescribed embodiments involve the specific example of carbon-dopedsilicon for NMOS applications, the skilled artisan will appreciate thatthe methods described herein have application to a variety ofsemiconductor applications where selective formation of a layer isdesired but etchants can interfere with desired properties of thedeposited layer.

Deposition methods exist that are useful for making a variety ofsubstitutionally doped single crystalline silicon-containing materials.For example, it is possible to perform in situ substitutional carbondoping of crystalline silicon by performing the deposition at arelatively high rate using trisilane (Si₃H₈) as a silicon source and acarbon-containing gas or vapor as a carbon source. Carbon-dopedsilicon-containing alloys have a complementary nature to silicongermanium systems. The degree of substitutional doping is 70% orgreater, expressed as the weight percentage of substitutional carbondopant based on the total amount of carbon dopant (substitutional andnon-substitutional) in the silicon. Techniques for forming carbon-dopedsilicon-containing materials have overcome several challenges, includingthe large lattice mismatch between carbon and silicon, the lowsolubility of carbon in silicon, and the tendency of carbon-dopedsilicon to precipitate. Additional details relating to in situsubstitutional carbon doping of crystalline silicon are provided in U.S.patent application Ser. No. 11/343,275 (filed 30 Jan. 2006; AttorneyDocket ASMEX.511A).

The term “silicon-containing material” and similar terms are used hereinto refer to a broad variety of silicon-containing materials includingwithout limitation silicon (including crystalline silicon), carbon-dopedsilicon (Si:C), silicon germanium, and carbon-doped silicon germanium(SiGe:C). As used herein, “carbon-doped silicon”, “Si:C”, “silicongermanium”, “carbon-doped silicon germanium”, “SiGe:C” and similar termsrefer to materials that contain the indicated chemical elements invarious proportions and, optionally, minor amounts of other elements.For example, “silicon germanium” is a material that comprises silicon,germanium and, optionally, other elements, for example, dopants such ascarbon and electrically active dopants. Terms such as “Si:C” and“SiGe:C” are not stoichiometric chemical formulas per se and thus arenot limited to materials that contain particular ratios of the indicatedelements. Furthermore, terms such as Si:C and SiGe:C are not intended toexclude the presence of other dopants, such that a phosphorous andcarbon-doped silicon material is included within the term Si:C and theterm Si:C:P. The percentage of a dopant (such as carbon, germanium orelectrically active dopant) in a silicon-containing film is expressedherein in atomic percent on a whole film basis, unless otherwise stated.

It is possible to determine the amount of carbon substitutionally dopedinto a silicon-containing material by measuring the perpendicularlattice spacing of the doped silicon-containing material by x-raydiffraction, then applying Vegard's Law by performing a linearinterpolation between single crystal silicon and single crystal carbon(diamond). For example, it is possible to determine the amount of carbonsubstitutionally doped into silicon by measuring the perpendicularlattice spacing of the doped silicon by x-ray diffraction, and thenapplying Vegard's law. Additional details on this technique are providedin the Hoyt article. It is possible to determine the total carboncontent in the doped silicon by secondary ion mass spectrometry(“SIMS”). It is possible to determine the non-substitutional carboncontent by subtracting the substitutional carbon content from the totalcarbon content. It is possible to determine the amount of other elementssubstitutionally doped into other silicon-containing materials in asimilar manner.

“Substrate,” as that term is used herein, refers either to the workpieceupon which deposition is desired, or the surface exposed to one or moredeposition gases. For example, in certain embodiments the substrate is asingle crystal silicon wafer, a semiconductor-on-insulator (“SOI”)substrate, or an epitaxial silicon surface, a silicon germanium surface,or a III-V material deposited upon a wafer. Workpieces are not limitedto wafers, but also include glass, plastic, or other substrates employedin semiconductor processing. As discussed in U.S. Pat. No. 6,900,115,the entire disclosure of which is hereby incorporated by referenceherein, a “mixed substrate” is a substrate that has two or moredifferent types of surfaces. For example, in certain applications amixed substrate comprises a first surface having a first surfacemorphology and a second surface having a second surface morphology. Incertain embodiments, carbon-doped silicon-containing layers areselectively formed over single crystal semiconductor materials whileminimizing, and more preferably avoiding, deposition over adjacentdielectrics or insulators. Examples of dielectric materials includesilicon dioxide (including low dielectric constant forms such ascarbon-doped and fluorine-doped oxides of silicon), silicon nitride,metal oxide and metal silicate. The terms “epitaxial”, “epitaxially”“heteroepitaxial”, “heteroepitaxially” and similar terms are used hereinto refer to the deposition of a crystalline silicon-containing materialonto a crystalline substrate in such a way that the deposited layeradopts or follows the lattice constant of the substrate. Epitaxialdeposition is generally considered to be heteroepitaxial when thecomposition of the deposited layer is different from that of thesubstrate.

Even if surfaces are made from the same elements, the surfaces areconsidered different if the morphologies (crystallinity) of the surfacesare different. The processes described herein are useful for depositingsilicon-containing films on a variety of substrates, but areparticularly useful for mixed substrates having mixed surfacemorphologies. Such a mixed substrate comprises a first surface having afirst surface morphology and a second surface having a second surfacemorphology. In this context, “surface morphology” refers to thecrystalline structure of the substrate surface. Amorphous andcrystalline are examples of different morphologies. Polycrystallinemorphology is a crystalline structure that consists of a disorderlyarrangement of orderly crystals and thus has an intermediate degree oforder. The atoms in a polycrystalline material are ordered within eachof the crystals, but the crystals themselves lack long range order withrespect to one another. Single crystal morphology is a crystallinestructure that has a high degree of long range order. Epitaxial filmsare characterized by a crystal structure and orientation that isidentical to the substrate upon which they are grown, typically singlecrystal. The atoms in these materials are arranged in a lattice-likestructure that persists over relatively long distances (on an atomicscale). Amorphous morphology is a non-crystalline structure having a lowdegree of order because the atoms lack a definite periodic arrangement.Other morphologies include microcrystalline and mixtures of amorphousand crystalline material. “Non-epitaxial” thus encompasses amorphous,polycrystalline, microcrystalline and mixtures of the same. As usedherein, “single-crystal” or “epitaxial” are used to describe apredominantly large crystal structure having a tolerable number offaults therein, as is commonly employed for transistor fabrication. Thecrystallinity of a layer generally falls along a continuum fromamorphous to polycrystalline to single-crystal; a crystal structure isoften considered single-crystal or epitaxial, despite low densityfaults. Specific examples of mixed substrates include without limitationsingle crystal/polycrystalline, single crystal/amorphous,epitaxial/polycrystalline, epitaxial/amorphous, singlecrystal/dielectric, epitaxial/dielectric, conductor/dielectric, andsemiconductor/dielectric. The term “mixed substrate” includes substrateshaving more than two different types of surfaces. Methods describedherein for depositing silicon-containing films onto mixed substrateshaving two types of surfaces are also applicable to mixed substrateshaving three or more different types of surfaces.

When grown into recessed source/drain areas, tensile strainedcarbon-doped silicon films (Si:C films) provide a tensile strainedsilicon channel with enhanced electron mobility, particularly beneficialfor NMOS devices. This advantageously eliminates the need to provide arelaxed silicon germanium buffer layer to support the strained siliconlayer. In such applications, electrically active dopants areadvantageously incorporated by in situ doping using dopant sources ordopant precursors. High levels of electrically active substitutionaldoping using phosphorous also contribute to tensile stress. Preferredprecursors for electrical dopants are dopant hydrides, including n-typedopant precursors such as phosphine, arsenic vapor, and arsine.Silylphosphines, for example (H₃Si)_(3-x)PR_(x), and silylarsines, forexample, (H₃Si)_(3-x)AsR_(x), where x=0, 1 or 2 and R_(x)═H and/ordeuterium (D), are alternative precursors for phosphorous and arsenicdopants. Phosphor and arsenic are particularly useful for doping sourceand drain areas of NMOS devices. SbH₃ and trimethylindium arealternative sources of antimony and indium, respectively. Such dopantprecursors are useful for the preparation of preferred films asdescribed below, preferably boron-, phosphorous-, antimony-, indium-,and arsenic-doped silicon, Si:C, silicon germanium and SiGe:C films andalloys.

Selective Epitaxial Formation of Tensile Strained Si:C Films.

Techniques have now been developed for selectively forming a tensilestrained Si:C film in exposed semiconductor windows, such as recessedsource/drain regions of a mixed substrate. For example, it is possibleto accomplish such selective formation by (a) blanket depositing a Si:Cfilm over a mixed substrate using trisilane as a silicon precursor, and(b) selectively etching the resulting non-epitaxial layer that is formedover the insulator portion of the mixed substrate. Steps (a) and (b) areoptionally repeated cyclically until a target epitaxial film thicknessover the recessed source/drain regions is achieved.

It is possible to form recessed source/drain regions by dry etching withsubsequent HF cleaning and in situ anneal. In embodiments wherein a dryetch is used, deposition of a selectively grown, thin (betweenapproximately 1 nm and approximately 3 nm) silicon seed layer helpsreduce etch damage. A seed layer also helps to cover damage caused byprior dopant implantation processes. In an example embodiment, such aseed layer might be selectively deposited using simultaneous provisionof HCl and dichlorosilane at a deposition temperature between about 700°C. and about 800° C.

In accordance with preferred embodiments a cyclical blanket depositionand etch process is illustrated in the flowchart provided in FIG. 1, andin the schematic illustrations of the partially formed semiconductorstructures illustrated in FIG. 2 though FIG. 5D. While illustrated foruse with Si:C deposition in recessed source/drain regions, it will beappreciated that the techniques described herein are advantageous forselective formation of epitaxial films in other circumstances, such ason active area islands surrounded by field isolation prior to any gatedefinition and without recessing.

In particular, FIG. 1 illustrates that a mixed substrate havinginsulator regions and recessed source/drain regions is placed in aprocess chamber in operational block 10. FIG. 2 provides a schematicillustration of an exemplary mixed substrate that includes a patternedinsulator 110 formed in a semiconductor substrate 100, such as a siliconwafer. The illustrated insulator 110, in the form of oxide-filledshallow trench isolation (STI), defines field isolation regions 112 andis adjacent recessed source/drain regions 114 shown on either side of agate electrode 115 structure. Note that the gate electrode 115 overliesa channel region 117 of the substrate. Together, the channel 117, sourceand drain regions 114 define a transistor active area, which istypically surrounded by field isolation 112 to prevent cross-talk withadjacent devices. In other arrangements, multiple transistors can besurrounded by field isolation. In one case, the top of the gatestructure 115, can be capped with dielectric, as illustrated. Thissurface then behaves similarly to the field regions 110 with respect tothe deposition there over, and the conditions that maintain selectivityin the field region will also apply to the top of the gate. In the casethat that the gate 115 is not capped with a dielectric, then the surfaceof the gate has the potential to grow polycrystalline material whichthen can be removed through in-situ etching of polycrystalline material,but a different set of selectivity conditions (pressure, gas flow, etc)would apply, compared to those used to ensure no residualpolycrystalline material on the field 110.

As indicated by operational block 20 in FIG. 1, and as illustratedschematically in FIG. 3, a blanket Si:C layer is then deposited over themixed substrate using trisilane as a silicon precursor. This results inamorphous or polycrystalline (non-epitaxial) deposition 120 over oxideregions 112, and lower epitaxial deposition 125 and sidewall epitaxialdeposition 130 over the recessed source/drain regions 114. Note that“blanket deposition” means that net deposition results over both theamorphous insulator 110 and the single crystal regions 114 in eachdeposition phase. While lack of etchant (e.g., lack of halides) ispreferred in the blanket deposition, in which case the deposition canalso be considered “non-selective,” some amount of etchant might bedesirable to tune the ratio of deposited thickness over the variousregions, as discussed in more detail below. In case such small amountsof etchant are desirable, the deposition process may be partiallyselective but nevertheless blanket, since each deposition phase willhave net deposition over both the insulator 110 and single crystalregion 114.

The regions of amorphous or polycrystalline deposition 120 and thesidewall epitaxial deposition 130 are then selectively etched in anoperational block 30 (FIG. 1), thus resulting in the structure that isschematically illustrated in FIG. 4. Preferably, little or noepitaxially deposited Si:C is removed from the lower epitaxial layer 125in the recessed source/drain regions 114 during the selective etch. Asdiscussed in more detail below, the vapor etch chemistry preferablycomprises a halide (e.g., fluorine-, bromine- or chlorine-containingvapor compounds), and particularly a chlorine source, such as HCl Cl₂.More preferably the etch chemistry also contains a germanium source(e.g., a germane such as monogermane (GeH₄), GeCl₄, metallorganic Geprecursors, solid source Ge) to improve etch rates. At the same timethat the non-epitaxial material 120 is selectively removed, someepitaxial material is left and some is removed. The sidewall epitaxiallayer 130 is of a different plane and is also more defective (due togrowth rate differential on the two surfaces) than the lower epitaxiallayer 125. Accordingly, the sidewall epitaxial layer 130 is more readilyremoved, along with the non-epitaxial material 120. Thus, each cycle ofthe process can be tuned to achieve largely bottom-up filling of therecesses 114. In some arrangements, epitaxial material can be left bythe process even on the sidewalls if it is of good quality and does nothinder the goals of the selective fill.

This process is repeated until a target thickness of epitaxial Si:C filmthickness is achieved over the recessed source/drain regions 114, asindicated by decisional block 40 (FIG. 1), and as schematicallyillustrated in FIG. 5A (deposition of second cycle of blanket Si:C layer120) and FIG. 5B (etch of second cycle of Si:C layer to leave layer ofepitaxial Si:C with increased thickness of epitaxial layer 125 inrecessed source/drain regions 114). FIG. 5C illustrates the result offurther cycle(s) to leave epitaxial refilled source/drain regions 114,where the selective epitaxial layers 125 are roughly coplanar with fieldoxide 110. FIG. 5D illustrates the result of further cycle(s) to leaveepitaxial layers 125 selectively as elevated source/drain regions 114.

The selective formation process may further include addition cycles ofblanket deposition and selective etch back from over dielectric regions,but without carbon doping to form a capping layer. The capping layer canbe with or without electrical dopants. For example, the portion of theelevated source/drain regions 125 of FIG. 5D that is above the originalsubstrate surface (i.e., above the channel 117) can be carbon-free,since it does not contribute to the strain on the channel 117.

In an example embodiment, the deposited Si:C film optionally includes anelectrically active dopant, particularly one suitable for NMOS devices,such as phosphorous or arsenic, thereby allowing phosphorous-doped Si:Cfilms or arsenic doped Si:C films to be deposited (Si:C:P or Si:C:Asfilms, respectively). The Si:C film is preferably deposited with anamorphous-to-epitaxial growth rate ratio that is preferably betweenabout 1.0:1 and about 1.6:1, more preferably between about 1.0:1 andabout 1.3:1, and most preferably between about 1.0:1 and about 1.1:1,such that the film thickness over insulator and over the recessedsource/drain regions is about equal. Manipulating the amorphous (or moregenerally non-epitaxial) to epitaxial growth rate ratio advantageouslyenables manipulation of the facet angle at the interface between theamorphous and crystalline Si:C after the subsequent etch process, andalso minimizes etch duration for removal, relative to greaterthicknesses over the insulators. Preferably the amorphous regions of theSi:C deposition have little or no crystallinity (i.e., are predominantlyamorphous), thus facilitating the subsequent etch in such regions.Furthermore, minimizing the excess of non-epitaxial deposition bybringing the ratio of thickness close to 1:1 reduces the length of theetch phase needed to clear non-epitaxial deposition from the fieldregions (and optionally from the gate).

In a preferred embodiment, the Si:C film is selectively etched from themixed substrate using an in situ chemical vapor etching technique. Thechemical vapor etching technique is optionally performed simultaneouslywith a brief temperature spike. In one embodiment, the temperature spikeis conducted using the process described in U.S. Patent ApplicationPublication 2003/0036268 (filed 29 May 2002; Attorney DocketASMEX.317A). As described therein, using a single wafer epitaxialdeposition tool with radiant heating through cold quartz or otherwisetransparent walls, a temperature spike can employ full power to theupper lamps for a short duration (for example, for about 12 to about 15seconds) while decoupling the power ratio for the lower lamps. In thisway, the wafer temperature can rapidly ramp up while the susceptortemperature lags significantly. The wafer temperature preferablyincreases from the loading temperature by between about 100° C. andabout 400° C., and more preferably by between about 200° C. and about350° C. Because of the short duration of the temperature spike and etchphase, the wafer is allowed to cool before the susceptor gets a chanceto approach the peak temperature. In this way, it takes far less timefor the wafer to cycle in temperature, as compared to simultaneouslycycling the temperature of a more massive combination of wafer/susceptortogether. An example reactor for use with this temperature spiketechnique is the EPSILON® series of single wafer epitaxial chemicalvapor deposition chambers, which are commercially available from ASMAmerica, Inc. (Phoenix, Ariz.).

However, in another embodiment, to aid in maintaining highconcentrations of substitutional carbon and electrically active dopants,while at the same time minimizing temperature ramp/stabilization times,the etch temperature is preferably kept low. Using a low temperature forthe etch also reduces the likelihood that electrically active dopantatoms are deactivated during the etch. For example, etching with Cl₂ gasadvantageously allows the etch temperature to be reduced, thus helpingto maintain the substitutional carbon and electrically active dopants.

Low temperatures for the etch phase enables roughly matching depositionphase temperatures while taking advantage of the high dopantincorporation achieved at low temperatures. Etch rates can be enhancedto allow these lower temperatures without sacrificing throughput byincluding a germanium source (e.g. GeH₄ GeCl₄, metallorganic Geprecursors, solid source Ge) during the etch phase instead of flashramping the temperature to improve throughput.

“Isothermal” cyclical blanket deposition and etching, as used herein,means deposition and etching within ±50° C. of one another, preferablywithin ±10° C., and most preferably setpoint temperature is within ±5°C. for both steps. Advantageously, isothermal processing improvesthroughput and minimizes time for temperature ramping and stabilization.Similarly, both blanket deposition and etching process are preferably“isobaric,” i.e., within ±50 Torr of one another, preferably within ±20Torr. Isothermal and/or isobaric conditions facilitate better throughputfor avoiding ramp and stabilization times.

As illustrated in FIG. 1, the two-stage process of performing a blanketdeposition followed by a selective etch is optionally repeatedcyclically until a target epitaxial film thickness over the recessedsource/drain regions is achieved. Example process parameters aresummarized in Table A below, which lists both preferred operating pointsas well as preferred operating ranges in parentheses. As is evident fromTable A, the process conditions—such as chamber temperature, chamberpressure and carrier gas flow rates—are preferably substantially similarfor the deposition and the etch phases, thereby allowing throughput tobe increased. Thus, the example below employs isothermal and isobaricconditions for both phases of the cycle. Other parameters are used inmodified embodiments.

TABLE A process phase post- flash- stabilize deposit bake post-bakechamber purge of pre-bake temp cool and temp and pre- process etchantspike temp TABLE A atmosphere deposit Deposit gases stabilize (optional)stabilize time  5 15  5  5 6.5   12.5 (sec) (2.5-7.5)  (5-20) (2.5-7.5) (2.5-7.5) (3.0-10) (10-15) temp 550 550  550  550  550 temp 550 (° C.)(500-650) (500-650)  (500-650)  (500-650)  (500-650) spike (500-650)pressure  64 64 64 64  64 64    64 (Torr)  (50-200) (50-200) (50-200)(50-200)  (50-200)   (50-200)  (50-200) H₂/He    2.0   2.0   2.0   2.0   2.0 2.0    2.0 (slm) (0.5-20)  (0.5-20)   (0.5-20)   (0.5-20)  (0.5-20)  (0.5-20) (0.5-20)  Cl₂/HCl 200 200    200 (sccm)   (5-1000)   (5-1000)   (5-1000) Si₃H₈ 75 75 75 (mg/min) (50-200) (50-200)(50-200) CH₃SiH₃ 150  150  150  (sccm) (10-300) (10-300) (10-300) PH₃ 5050 50 (sccm) (10-200) (10-200) (10-200)

Using the parameters provided in Table A, it is possible to achieve netgrowth rates that are preferably between about 4 nm min⁻¹ and about 11nm min⁻¹, and more preferably between about 8 nm min⁻¹ and about 11 nmmin⁻¹, for epitaxial Si:C:P films that are selectively deposited inrecessed source/drain regions. It is also possible to achieve thinSi:C:P films with substitutional carbon content up to 3.6% as determinedby Vegard's Law, and with resistivities between about 0.4 mΩcm and about2.0 mΩcm. By manipulating the deposition conditions, it is possible toobtain other film properties.

During the etch process disclosed herein, epitaxial Si:C is etchedsignificantly slower than amorphous or polycrystalline Si:C in each etchphase (etch selectivity in the range of 10:1-30:1). Defective epitaxialmaterial is also preferentially removed in the etch phases. In apreferred embodiment, the cyclical deposition and etch processconditions are tuned to reduce or eliminate net growth on the oxidewhile achieving net growth in each cycle in the epitaxial recessedsource/drain regions. This cyclical process is distinguishable fromconventional selective deposition processes in which deposition andetching reactions occur simultaneously.

Tables B and C below give two examples of deposition and etch durationsand resultant thicknesses using a recipe similar to that of Table A. Therecipes are differently tuned to modulate both deposition and etch ratesby increasing the partial pressure of the Si₃H₈ and optimizing etchantpartial pressures.

TABLE B Deposition Phase Etch Phase Growth rate [nm/min] 28 13 α-etchrate [nm/min] Deposition time[s] 22 47.4 Minimum etch time[s] 60 %overetch 75.8 Effective etch time[s] Deposited α-thickness 10.27 16.43Removed α thickness [nm] [nm] Deposited epi-thickness 9.78 0.82 Removedc thickness [nm] per deposition step [nm] per etch step α/epi growthrate ratio 1.05 20 In-situ etch selectivity Purge (pre epi + post 25 25Purge (pre epi + post epi) [s] epi) [s] Final time/cycle[s] 122.8 Finalthk/cycle [nm]  8.96 Average growth rate [nm/min]  4.38

TABLE C Deposition Phase Etch Phase Growth rate [nm/min] 80 25 Etch rate[nm/min] Deposition time[s] 8 25.6 Minimum etch time[s] 30 % overetch33.28 Effective etch time[s] Deposited α-thickness 10.67 13.87 Removed αthickness [nm] [nm] Deposited epi-thickness 10.67 0.693 Removed cthickness [nm] per deposition step [nm] per etch step α/epi growth rateratio 1 20 In-situ etch selectivity Purge (pre epi + post 20 20 Purge(pre epi + post epi) [s] epi) [s] Final time/cycle[s] 61.3 Finalthk/cycle [nm]  9.977 Average growth rate [nm/min]  9.76

The process parameters provided in Table A indicate a Cl₂/HCl etchchemistry. In a modified embodiment, between about 20 sccm and about 200sccm of 10% GeH₄ is included in the etch chemistry as an etch catalyst.In certain embodiments, inclusion of a germanium source (e.g., a germanesuch as GeH₄, GeCl₄, metallorganic Ge precursors, solid source Ge) inthe etch chemistry advantageously enhances the etch rate and the etchselectivity. In addition, use of germanium as a catalyst alsoadvantageously allows lower etch temperatures to be used, and allows atemperature spike during etch to be omitted, as noted above indiscussion of isothermal processing. Additional information regardingdiffusion of germanium in amorphous, polycrystalline and singlecrystalline silicon and the subsequent etching of Ge rich siliconmaterials is provided in the literature; see for example, Mitchell etal., “Germanium diffusion in polysilicon emitters of SiGe heterojunctionbipolar transistors fabricated by germanium implantation”, J. of Appl.Phys, 92(11), pp. 6924-6926 (1 Dec. 2002), Wu et al., “Stability andmechanism of selective etching of ultrathin Ge films on the Si(100)surface,” Phys. Rev. B, 69 (2004), and Bogumilowicz et al., “Chemicalvapour etching of Si, SiGe and Ge with HCI; applications to theformation of thin relaxed SiGe buffers and to the revelation ofthreading dislocations upon chlorine adsorption,” Semicond. Sci. &Tech., no. 20, pp. 127-134, (2005).

FIG. 6 shows a graph of etch rate of amorphous regions of a carbon-dopedsilicon film as a function of HCl partial pressure in the etchchemistry, at a constant temperature of 600° C. By decreasing the H₂carrier flow, the partial pressure of HCl and GeH₄ is increased, therebysignificantly increasing the amorphous etch rate in certain embodiments.For example, FIG. 6 indicates that inclusion of 20 sccm of 10% GeH₄ inthe etch chemistry (symbols ▾ and ▴) results in substantially higheramorphous etch rates.

FIG. 7 shows a graph of etch rate and amorphous/epitaxial etch rateratio as a function of GeH₄ flow in the etch chemistry, at a constanttemperature of 600° C., a constant H₂ carrier flow of 2 slm, and aconstant chamber pressure of 64 Torr. Amorphous etch rates are indicatedby the “a-” prefix in the legend, epitaxial etch rates are indicated bythe “c-” prefix in the legend, and etch rate ratios are indicated by“ER” in the legend. Increasing the GeH₄ flow causes theamorphous/epitaxial etch rate ratio to increase to a point, beyond whichadditional GeH₄ reduces etch selectivity. For example, FIG. 7 indicatesthat an etch chemistry comprising 200 sccm HCl and approximately 30 to40 sccm of 10% GeH₄ produces an amorphous/epitaxial etch rate ratio thatcannot be obtained with lower or higher GeH₄ flow rates.

FIG. 8 shows a graph of etch rate of amorphous regions of a Si:C film asa function of chamber pressure for various GeH₄ flow rates in the etchchemistry, at a constant temperature of 550° C., a constant H₂ carrierflow of 2 slm, and a constant HCl etchant flow of 200 sccm. Byincreasing the chamber pressure beyond approximately 80 Torr, thedependence of the etch rate on the GeH₄ flow rate is reduced. However,by increasing the chamber pressure from about 64 Torr to about 80 Torrwhen 50 sccm of 10% GeH₄ is included in the etch chemistry, theamorphous etch rate is increased by a factor of about two.

FIG. 9 shows a graph of etch rate of amorphous regions of a carbon-dopedsilicon film as a function of reciprocal temperature, at a constantchamber pressure of 64 Torr, a constant H₂ carrier flow of 2 slm, aconstant HCl etchant flow of 200 sccm, and a constant GeH₄ etchant flowof 50 sccm of 10% GeH₄. The absolute etch rates are very high for thesechemicals even at very low temperatures.

FIG. 10 shows a graph of thickness of amorphous regions of acarbon-doped silicon film as a function of accumulated etch time, at aconstant chamber pressure of 64 Torr, a constant chamber temperature of550° C., a constant H₂ carrier flow of 2 slm, and a constant HCl etchantflow of 200 sccm. The slopes of the lines plotted in FIG. 10 correspondto the etch rate of the amorphous Si:C film. As indicated, the etch ratein the center of the deposited film is greater than the etch rate at theedge of the deposited film. Therefore, in a preferred embodiment thewafer is “overetched” to increase the likelihood that amorphous Si:C isremoved from the slower-etching wafer edges. By extrapolating the linesplotted in FIG. 10 to the y-axis, it is possible to estimate the initialamorphous film thickness and growth rate. Likewise, by extrapolating thelines plotted in FIG. 10 to the x-axis, it is possible to estimate thetime required to etch the amorphous material entirely. FIG. 10illustrates that an etch rate of approximately 140 Å min⁻¹ is obtainedwith the provided process parameters.

FIG. 11 shows a graph of etch rate of amorphous regions of acarbon-doped silicon film deposited on a wafer as a function of radialposition on the wafer, at a constant chamber temperature of 550° C., aconstant chamber pressure of 64 Torr, a constant H₂ carrier flow of 2slm, and a constant HCl etchant flow of 200 sccm. FIG. 11 indicates thatthe etch rate is slightly slower at the wafer edge than the wafercenter.

FIG. 12 shows a graph of thickness of amorphous regions of acarbon-doped silicon film deposited on a wafer as a function of radialposition on the wafer for various etch cycle durations, at a constantchamber temperature of 550° C., a constant chamber pressure of 80 Torr,a constant H₂ carrier flow of 2 slm, a constant HCl etchant flow of 200sccm, and a constant GeH₄ etchant flow of 6.5 sccm.

FIG. 13 shows a graph of thickness of amorphous regions of acarbon-doped silicon film deposited on a wafer as a function of radialposition of the wafer for various GeH₄ etchant etch chemistries andvarious etch cycle durations. As illustrated in FIG. 13, longer etchcycles and higher GeH₄ flow rates leads to more nonuniform etching. In amodified embodiment, this effect is compensated for by providing a finaletch cycle of extended duration, thereby providing sufficient “overetch”to remove amorphous Si:C remaining at the center of the wafer.Accordingly, it is desirable to deposit and etch relatively smallthicknesses in each cycle, preferably between about 1 nm/cycle and 10nm/cycle, more preferably between about 2 nm/cycle and 5 nm/cycle. Asnoted above, conditions similar to Table A have been used to achieve netdeposition rates of 4-11 nm/min.

FIG. 14 is a photograph illustrating an example partially formedcarbon-doped silicon structure created by performing one depositioncycle and one etch cycle on a patterned substrate. As illustrated,crystalline Si:C:P is present over an epitaxial substrate region, whileamorphous Si:C:P is present over oxide. An amorphous pocket is presentat the amorphous/epitaxial interface because deposition occurs atdifferent growth rates depending on the exposed crystallographicorientation. In the structure illustrated in FIG. 14, the ratio ofamorphous etch rate to epitaxial etch rate is over 20. FIG. 16 is aphotograph illustrating an example partially formed carbon-doped siliconstructure created by performing multiple deposition and etch cycles on apatterned substrate. As compared to FIG. 14, substantially all amorphousSi:C:P has been removed from oxide surfaces, resulting in pseudoselective epitaxial formation. FIG. 17 illustrates an atomic forcemicroscopy analysis of a epitaxial carbon-doped silicon film that hasbeen selectively deposited using certain of the techniques disclosedherein.

FIG. 15 shows a graph of element concentration as a function of depthfor an example partially formed carbon-doped silicon film formed usingcertain of the techniques disclosed herein. As illustrated, relativelymodest levels of germanium are incorporated into the Si:C film due toGeH₄ use during the etch phase. Preferably, the germanium incorporationis less than about 5 atomic %, more preferably less than about 2 atomic%, and most preferably less than about 1 atomic %.

FIG. 18 shows x-ray diffraction rocking curves of carbon-doped siliconfilms deposited using certain of the techniques disclosed herein. Thecurves indicate different quantities of deposition/etch cycles, andcorrespond to increasing monomethylsilane (“MMS”) flow rates, whichcorresponds to higher substitutional C concentrations in the siliconepitaxial film.

The techniques disclosed herein for selective epitaxial deposition ofSi:C films provide several advantages over conventional techniques. Forexample, cyclical removal of polycrystalline or amorphous Si:C frominsulator regions helps to improve the interface between the amorphousSi:C and the epitaxial Si:C. In particular, the cyclical process allowsepitaxial growth to occur in interface regions where non-epitaxialgrowth would otherwise occur. Furthermore, in embodiments wherein thetemperature spike during etch is omitted, such that the etch cycle isconducted at a temperature that is equal to, or only slightly elevatedfrom, the deposition cycle, lower temperatures lead to many advantages.Throughput is improved by minimizing temperature (and/or pressure) rampand stabilization time. Deposition temperatures can still be low enoughto achieve high (e.g., 1.0-3.6 at C %) substitutional carbon content,and a large portion of the substitutional carbon and electrically activedopants remain in place during the etch, thus resulting in highsubstitutional carbon and dopant concentrations in the resulting film.

Several features contribute to the high throughput. For example, use oftrisilane has been found to improve deposition rates at very lowtemperatures, thus minimizing the duration of deposition phases withoutsacrificing, e.g., high substitutional dopant concentrations asdeposited that result from lower temperatures and higher depositionrates. The sequences and choice of precursors also facilitates largelyor even wholly amorphous deposition over amorphous insulating regionswhile also leading to relatively uniform thicknesses (thickness ratiosof less than 1.6:1) over both single-crystal and amorphous regions,minimizing overall etch time during the etch phases.

Scope of the Invention

While the foregoing detailed description discloses several embodimentsof the present invention, it should be understood that this disclosureis illustrative only and is not limiting of the present invention. Itshould be appreciated that the specific configurations and operationsdisclosed can differ from those described above, and that the methodsdescribed herein can be used in contexts other than fabrication ofsemiconductor devices.

1. A method of selectively forming epitaxial semiconductor material,comprising: blanket depositing semiconductor material to form epitaxialmaterial over single-crystal semiconductor regions of a substrate and toform non-epitaxial material over insulating regions of the substrate;and selectively removing the non-epitaxial material from over theinsulating regions by exposing the blanket deposited semiconductormaterial to an etch chemistry including a halide source and a germaniumsource; and repeating blanket depositing and selectively removing atleast once.
 2. The method of claim 1, wherein blanket depositingcomprises flowing trisilane into a chemical vapor deposition chamberhousing the substrate.
 3. The method of claim 1, wherein blanketdeposition comprises depositing between about 1 nm and 10 nm in eachcycle.
 4. The method of claim 1, wherein blanket depositing andselectively removing are isothermal and isobaric processes conducted ina chemical vapor deposition chamber.
 5. The method of claim 1, whereinthe semiconductor regions comprise recessed regions.
 6. The method ofclaim 5, wherein the epitaxial material in the recessed regions exertsstrain on adjacent regions of the substrate.
 7. The method of claim 1,wherein the semiconductor material comprises carbon-doped silicon.
 8. Amethod of forming silicon-containing material in selected locations on asubstrate, the method comprising: providing a substrate having exposedwindows of single-crystal semiconductor among field isolation regions;blanket depositing silicon-containing material over the windows ofsingle-crystal material and the field isolation regions by flowingtrisilane over the substrate; and selectively removing thesilicon-containing material from over the field isolation regions; andrepeating blanket depositing and selectively removing in a plurality ofcycles.
 9. The method of claim 8, wherein blanket depositing thesilicon-containing material comprises flowing a carbon source vapor withtrisilane over the substrate and the silicon-containing materialcomprises carbon-doped silicon.
 10. The method of claim 9, whereinselectively removing comprises etching non-epitaxial material from overthe field isolation regions at a faster rate than removing epitaxialmaterial from over the windows of single-crystal semiconductor.
 11. Themethod of claim 10, wherein the windows of single-crystal semiconductorcomprise recesses, and selectively removing further comprises removingepitaxial material from sidewalls of the recesses while leavingepitaxial material at bottoms of the recesses.
 12. The method of claim8, wherein selectively removing comprises exposing the substrate to ahalide etchant and a germanium source.
 13. The method of claim 8,wherein the windows of single-crystal semiconductor comprise recessesrelative to upper surfaces of the field isolation regions.
 14. Themethod of claim 8, wherein blanket depositing and selectively removingare conducted under isothermal and/or isobaric conditions.
 15. Themethod of claim 8, wherein blanket depositing and selectively removingproduces a net growth rate of the silicon-containing material of betweenabout 4 nm/min and 11 nm/min.
 16. A method of selectively formingepitaxial semiconductor material, comprising: providing a substrate withinsulating regions and semiconductor windows formed therein; blanketdepositing non-epitaxial semiconductor material over the insulatingregions and epitaxial semiconductor material over the semiconductorwindows; and selectively etching the non-epitaxial semiconductormaterial from over the insulating regions while leaving at least someepitaxial semiconductor material in the semiconductor windows byexposing the substrate to a chlorine source and a germanium source; andrepeating blanket depositing and selectively removing in a plurality ofcycles.
 17. The method of claim 16, wherein the semiconductor windowscomprises recesses below upper surfaces of the insulating regions andrepeating comprises filling the recesses with epitaxial semiconductormaterial.
 18. The method of claim 17, wherein depositing comprisesfilling the recesses with carbon-doped silicon to form recessedsource/drain structures.
 19. The method of claim 18, wherein depositingfurther comprises in situ supplying electrical dopants to the recessedsource/drain structures.
 20. The method of claim 18, wherein depositingcomprises forming a carbon-doped silicon and further comprises forming acarbon-free capping layer over the carbon-doped silicon.
 21. The methodof claim 16, wherein the semiconductor windows comprises recesses belowupper surfaces of the insulating regions and the epitaxial semiconductormaterial in the recesses exerts lateral tensile strain on an adjacentregion.
 22. The method of claim 21, wherein selectively removing furthercomprises removing epitaxial material from sidewalls of the recesseswhile leaving epitaxial material at bottoms of the recesses.
 23. Themethod of claim 16, wherein in each depositing step, a thickness ratioof non-epitaxial semiconductor material over the insulating regions tothe epitaxial semiconductor material over the semiconductor windows isless than about 1.6:1.
 24. The method of claim 23, wherein the ratio isbetween about 1.0:1 and 1.3:1.
 25. The method of claim 16, whereinblanket depositing comprises flowing trisilane into a chemical vapordeposition chamber housing the substrate.